In a typical Code Division Multiple Access (CDMA) cellular network, such as a second generation (2G) CDMA communication network, transcoders are located in each of an originating mobile station (MS), a terminating MS, and a base station (BS), and in particular in a base station controller (BSC), serving each of the originating and terminating MSs. The transcoder of the BS serving the originating MS receives compressed voice packets from the MS and converts the voice packets to pulse code modulated (PCM) signals for transmission through a circuit switched core network included in the cellular network. The BS then transmits the PCM signals upstream through the circuit switched core network and, via the core network, to a Public Switched Telephone Network (PSTN) coupled to the operator's cellular network. Similarly, PCM signals received by a 2G CDMA cellular network from a PSTN that are intended for the terminating MS are transmitted as PCM signals through the circuit switched core network to the BS servicing the terminating MS, where the transcoder in the BS converts the PCM signals to compressed voice packets. The BS then transmits the compressed voice packets to the terminating MS.
The development of next generation CDMA networks, such as a cdma2000 cellular network, have allowed system operators to install Packet Switched core networks in place of, or in parallel with the circuit switched core networks, thereby permitting data packets to be transmitted through the cellular networks in place of, or in parallel with circuit switched signals. The installation of such Packet Switched core networks permits a system operator to transmit voice data as a compressed voice packet through the Packet Switched core network, rather than transmit voice data as PCM signals through the circuit switched core network. To facilitate transmission of voice through a cellular network in a data packet format, operators of cdma2000 cellular networks have expressed an interest in relocating the transcoders closer to the PSTN. In addition, a relocation of transcoders to a more centralized location deeper in the cellular network can reduce system costs by providing a more centralized transcoder function, as opposed to a widely distributed, BS-based, transcoder function, and by permitting voice services to be transported over more of a backhaul network in a compressed format as opposed to an uncompressed format.
However, when a legacy CDMA communication system is upgraded with a relocated transcoder function, a result may be a provision of a transcoding function in each of a core network and a BS. Furthermore, some systems may service mobile stations that are not capable of transmitting voice in a format compatible for transmission over a Packet Switched core network, thus necessitating a transcoding function in a BS. A result is that multiple transcoding functions may reside along a voice signal's path, which multiple transcoding functions may each support a same or different bearer format type. In order to determine an appropriate bearer format type for a communication session involving an originating mobile station (MS) and a terminating MS, the transcoding capabilities of the originating and terminating MSs, as well as the transcoding capabilities of any transcoding functions residing along a voice bearer path associated with the MSs, must be made known to a decision-making entity.
In addition, each time voice is re-encoded/re-decoded by a transcoder a quality of the voice degrades. As a result, the standards underlying the next generation CDMA networks further allow for, but do not specify implementation of, transcoder-free operation (TrFO), wherein voice is transmitted through the cellular network without application to the voice, by the network, of any transocoder function. However, in order to for a cellular network to engage in transcoder-free operation or to engage appropriate transcoders into operation, the network must determine the most appropriate bearer format type to be supported by each of the Network Elements involved in the call, including an originating MS and the terminating MS for the case of mobile-to-mobile calls.
Therefore, there exists a need for a method and apparatus for determining the bearer format type capabilities of each MS involved in a communication session and further of the transcoding functions within Network Elements (for example, Media Gateways) located along a voice bearer path associated with the call session.